Experiments in the past year have succeeded in establishing the complete amino acid sequence of the lambda phage repressor and the nucleotide sequence of its corresponding gene as well as the nucleotide sequence of the cro gene. A repressor monomer is a protein containing two domains separated by a connector 42 amino acids long. The domains are separated by proteolytic cleavage, and each domain has been crystallized as has the cro protein. Recombinant DNA techniques have been used to produce special strains that make large amounts of these and other regulatory proteins. The structure and function of these molecules are currently under continuing investigation.